Downloaded diagram from Tanya, but it uses a different IC - TA8427K
Supply to IC is 28V.
My set reads 15.
I will check it again tomorrow.
This diagram shows a transistor Q451 in the supply.
Maybe it's the culprit - if there is one in the set.

Let me amend my previous statement. A large cap around 2200 uF couples the vertical signal to the yoke. At the cold end of the yoke there is a small value resistor used for current feedback to the vertical circuit. The coupling cap and the resistor are worth checking. The input to the vertical IC on pin 7 should be a believable linear ramp. As others have stated, a partial vertical raster is often caused by a bad pump up capacitor.

The datasheet said the IC can handle a supply voltage of up to 35 volts with a minimum being 10 volts.

C306 is the pump up cap and D301 is the pump up diode in that diagram.

D301 was it!

Why did I not replaced it, even though it measured ok?!...
When I gave it solder heat the picture opened up full screen.
Replaced it with a 1N4004 - it has been a while since I had trouble with such an item.